There is no "legal" limit on boats over 20 feet. But as a practical and safety matter, I would say 8 to 10, possibly 12. I know its a big boat, but trying to keep tabs on everyone, and staying in total control of your "ship", that seems more than enough "fun". You are considered the master, and your the one they will be pointing fingers at if something goes wrong. The only legality is if someone drowns or is injured, and if they can pin carelessness or recklessness on you. And you know they will try.
Another thing to consider is how everyone is going to watch the fireworks. Is everyone going to stand in the cockpit? Will people be out on the deck? Would you want people out on the deck at night? Then it seems how ever many people can comfortably sit in the cockpit is the maximum capacity you would want aboard.